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need help with ati driver and xrandr dual head setup

Hello Folks,

I'm new at configuring Linux so please have some patience with me. I'm trying to set up a dual head display with one desktop stretched out on both screens. I have an ATI X800 and due to recommendations from this guide https://help.ubuntu.com/community/RadeonDriver I'm trying to achieve this with the open source ati driver and xrandr.

It's actually sort of working, I do have the desktop stretched out over both screens and I can drag windows from one screen to another. The thing is that on the right side of the right screen these is a wide stripe, that doesn't show the desktop backgroud, but rather some kind of "shadows" of the windows being dragged over it. Can anyone please help me a bit?

It's actually sort of working, I do have the desktop stretched out over both screens and I can drag windows from one screen to another. The thing is that on the right side of the right screen these is a wide stripe, that doesn't show the desktop backgroud, but rather some kind of "shadows" of the windows being dragged over it. Can anyone please help me a bit?

Are you using compiz at the time? That card has a maximum 3D texture size of 2048, which means that any texture larger than 2048x2048 (such as your wallpaper image at 2560x1024) will not display properly paste the 2048th pixel.

You could try disabling gnome/xfce/kde from drawing the wallpaper and simply let compiz draw it's own. Then you wouldn't see this problem.

I can't reproduce the same error anymore and I'm getting inconsistent results with every reboot. Sometimes both screens are showing either only the left half or only the right half of the desktop, depending if I use the --right-of or the --left-of option. Sometimes one of the screens shows both halves of the desktop squeezed down to a width of 1280 pixels. Sometimes my mouse doesn't get recognized. All this with the same configuration.

Then I used Gnome's System -> Preferences -> Screen Resolution and unchecked the "Clone Screens" options there and aligned the two screens next to each other. Right now the whole thing works without the graphical error I described in my OP, the desktop background tiles nicely. But now I can't enable the Visual Effects in System -> Preferences -> Appearance.

I feel so lost in this. I don't know if I'm using compiz, how can I find it out? And well, here is my xorg log marcell.missura.de/Xorg.0.log

If you start compiz before you extend your screen to the second monitor, you should not get any error messages when trying to start it. If you extend your screen first, and then try to start compiz, you will get an error.

If you want to see if you are currently using compiz, you can bring up a terminal and run 'ps ax | grep compiz'. That will show you the compiz process, if it's running.

What can I do about this? I would really like to keep the 1280x1024 resolution on both screens and not go down to 1024x768. Is maybe the xrandr approach not the right one for me?

Thanks
Cruz

xrandr is the *only* approach. And, even if there were others, you're still going to hit that hardware limit.

As a workaround, you could run two monitors in a virtual vertical setup, with one above or below the other. If they are both 1280x1024, that would give you a total of 1280x2048, which is within the limits of the video card.

Another option is to simply bypass that check by running this command: 'mkdir -p ~/.config/compiz; echo SKIP_CHECKS=yes >> ~/.config/compiz/compiz-manager'

However, your wallpaper beyond the 2048th pixel will be corrupted, and any windows past that point will leave a neat looking trail, just as you described in your first post. Disabling gnome from drawing it's own desktop, and using compiz to draw the wallpaper, will improve things a little. You can do that via gconf-editor --> apps/nautilus/preferences, disable "show_desktop".

Why is xrandr the only approach? In this guide https://help.ubuntu.com/community/RadeonDriver there are two alternative approaches explained, MergedFB and Xinerama. It must be possible to circumvent the hardware limit somehow, since the dual screen works perfectly on Win XP.